This invention relates generally to a document reproduction system and, more particularly to an optical system in which a document is moved along an object plane through an exposure zone and a reflected image is projected onto a moving flat photoreceptor at a rate faster than the rate of movement of the photoreceptor and in the direction opposite such photoreceptor movement.
Precession scan systems which expose images on a photoreceptor at a faster rate than the photoreceptor movement known in the art. U.S. Pat. No. 3,454,335 (Caldwell) assigned to the same assignee as the present invention, discloses a method of projecting images on microfilm cards which are moved past a stationary lens and mirror system onto a drum photoreceptor. The drum and microfilm are moved at the same speed but the image is precessed, (i.e. moved in a direction opposite to the drum motion) onto the drum surface by means of a slit aperture which moves in a direction opposite that of the drum. This combined slit and drum rotation exposes the drum at a speed (rate) that is greater than the surface speed of the drum. This scanning system demonstrates two of the advantages gained by image precession: the process speed of the system can be set at a lower speed for equivalent copy rate (copies per minute) and the gap ordinarily present between images (due to the return time of the scan optics) can be reduced or eliminated if desired. Scan return velocities for a given copy rate can also be minimized for a given copy rate thus reducing scan accelerations, forces and vibrations. A problem with such a system, however, is that defocusing errors may result in developed images unacceptable for some systems. The errors are introduced because the image reflected from the drum mirror to the slit does not maintain perpendicularity to a tangential line at the drum surface.
This defocusing problem is addressed in U.S. Pat. No. 4,232,960 (Glab) assigned to the same assignee as the present invention. Glab solves the problem of field tilt in his particular scan system, by using linear and rotational motions of optical elements located near the drum surface to scan the image onto the drum at the angle corresponding to the angle of reflection of the image from the platen.
Another prior art device which utilizes precession onto a drum photoreceptor is disclosed in U.S. Pat. No. 3,650,621 (Lewis) also assigned to the same assignee as the present invention. Lewis discloses a system wherein an imaging device is moved in an arcuate path which is in a direction opposite to drum rotation to scan document on a curved platen onto the drum.
Precession onto a flat photoreceptor surface introduces different problems requiring a different set of solutions. U.S. Pat. No. 3,754,822 (Melrose) also assigned to the same assignee as the present invention, discloses a scan system wherein a platen and flat photoreceptor move synchronously in the same direction and the scan optics move in the opposite direction at a suitable speed. The disclosure and claims are limited to the specific system disclosed and do not address problems in systems which, for example, have a stationary object or which have a lens movement separate from that of a scan lamp movement. In higher volume copiers, a preferred method of scanning has been to utilize a full-rate, half-rate mirror system which maintains proper object-to-lens distance during the course of travel. Such a scan system is relatively heavy, however, and the above-mentioned flyback inertial problems constitute an important limitation on the speed of the surface.
Precession onto a flat photoreceptor surface is also disclosed in copending U.S. Application Ser. No. 190,160, filed Sept. 24, 1980, assigned to the same assignee as the present invention. In this application, an object is imaged onto a photoreceptor by an illumination scanning system which is moving at a faster rate than the photoreceptor.
Another variation of precessing an image onto a flat photoreceptor is disclosed in copending U.S. Application Ser. No. 237,032, filed Feb. 23, 1981, assigned to the same assignee as the present invention. In this application, a fixed document lying in an object plane is scanned by a modified dual rate mirror system where the second (slower) mirror is moving at less than half the speed of the first (scan) mirror thereby causing a resultant decrease in platen-to-lens distance. This decrease is compensated for, and image precession obtained, by moving a projection lens and associated mirror along the optical axis in a direction opposed to that of the photoreceptor.
None of the above-disclosed systems have utilized an image system wherein a document to be copied is moved along an object plane through an exposure zone. This type of system, referred to as a controlled velocity transport (CVT) system, is relatively efficient where a plurality of documents are stream fed past the exposure zone. Where, however, a number of copies of a single document are needed, there exists a recovery time factor associated with recirculating the document back to the start of scan position after the completion of an exposure cycle. One of the advantages of a precession imaging system as described in the above references is to relax return scan component velocity requirements. By analogy, a precession system can also be usefully employed in a CVT system to compensate for the time taken to return a document to a start of scan position. The present invention provides such a precession scheme by fixing in place a first scan mirror and by moving a lens and mirror arrangement along an optical path away from the object plane so as to project images onto a moving photoreceptor in a direction opposite to the photoreceptor movement. The platen to lens optical distance is held constant by moving a second mirror, placed between the scan mirror and lens, toward the scan mirror. The lens and second mirror are moved at rates derived according to one of the principles of the invention.
In one embodiment wherein documents are copied in both a stream fed mode and a repeat mode, an efficient dual speed document system is further disclosed.